[llvm] [Docs][RISCV] Document RISC-V vector codegen (PR #96740)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp commented:

Great! I don't see any problem here!
I think we need some unfamiliar eyes on this doc, because we may miss some important details since we all are familiar with RVV implementation.

https://github.com/llvm/llvm-project/pull/96740